The present invention generally relates to memory devices for use with computers and other processing apparatuses. More particularly, this invention relates to a non-volatile or permanent memory-based mass storage device using background scrubbing during power-down periods of the host system or during physical disconnects from the host system to identify faulty storage addresses.
NAND flash memory-based mass storage devices, and particularly solid state SATA drives, are becoming important players in the mass storage device market. Primary reasons for the rapid acceptance of these solid state drives (SSDs) are the extremely fast access times for data, along with low power requirements compared to earlier electromechanical hard disk drives (HDDs). As a rough estimate, the maximum power consumption of a 125 GB SSD is often in the range of about 1.5 to 2 Watts, depending on the integrated circuits (ICs) used. This energy efficiency opens up a number of possibilities for operating the drives that are not possible in the case of HDDs.
Of particular interest related to energy efficiency is the possibility of powering SSDs with batteries to allow house-keeping functions to be autonomously performed during offline periods. This capability can help to avoid traffic congestion during periods of usage by performing a surface scrub for bad blocks when the host system is powered off. The demand for surface scrubbing in SSDs originates in the inherent weaknesses of NAND flash memory with respect to data retention and write endurance. Both become progressively worse with every transition to a new process node. For example, interactions are encountered with smaller geometries in the form of proximity disturbances that can occur within a memory cell when a nearby cell is read or written to. In addition, exposure to high temperature and/or changes in temperature can lead to data loss due to recoverable bit errors. The latter can contribute to offline data loss, that is, a drive without problems during its last operation can suddenly develop problems even if it is powered off, and may fail without warning on the next attempt of operation. Moreover, in contrast to HDDs with an almost unlimited offline data retention, SSDs based on NAND flash technology typically are expected to retain data only for 6 months. Consequently, offline loss of data is often unpredictable since it occurs independent of the operation of the device with its host system.
More generally, the checking of data integrity in mass storage systems during periods of no-transfers is often referred to as disc scrubbing, as described by Schwartz et al., Modeling, Analysis, and Simulation of Computer and Telecommunications Systems, Proceedings of the IEEE Computer Society's 12th Annual International Symposium, 409 (Oct. 4-8, 2004). The underlying principle is to use idle periods of drives to check for bad blocks of memory and then rebuild the data in a different location. U.S. Pat. No. 5,632,012 to Belsan describes such a disk scrubbing system. U.S. Patent Application 2002/0162075 to Talagala describes disk scrubbing at the disk controller level wherein the disk controller reads back data during idle phases and generates a checksum that is compared to a previously stored checksum for the same data. Any disparity between the checksums of the area scanned is used to identify bad data and initiates rebuilding of the data at different addresses using redundancy mechanisms.
U.S. Pat. No. 6,292,869 to Gerchman et al. describes the interruption of self-timed refresh upon receiving a scrub command from the system to scrub memory arrays. U.S. Pat. No. 6,848,063 by Rodeheffer teaches memory scrubbing of very large memory arrays using timer-based scan rates, wherein the scan rate can be defined depending on the requirements of the system.
The above prior art does not disclose or suggest a self-contained mass storage device that can autonomously perform surface scrubbing.